In ink-jet recording methods, ink droplets are generally formed by using any of various ink discharge methods. Such ink-jet recording methods include applying a high voltage to form an electrostatic attraction or applying mechanical vibration or displacement to an ink using a piezoelectric element. Another ink-jet recording method heats an ink to generate bubbles and utilizes the resulting pressure. These methods provide ink droplets force to force ink droplets to fly and subsequently adhere as ink dots to a recording material, such as a recording paper, thereby performing the recording. Ink-jet recording methods typically generate less noise than other recording methods and allow high-speed printing or multi-color printing.
Ink-jet recording methods can be used to create images on various types of recording material, for example, plain paper, dedicated recording paper; and optical transparency sheets for slide or overhead projector (OHP). Other ink-jet recording methods utilize a color-separation plate for forming a positive plate for color printing or a color mosaic filter (CMF) for a color display device such as a liquid crystal device.
When a color image is formed by the ink-jet recording method, generally various colors can be developed using a subtractive color mixing method by overprinting inks of three primary colors (yellow (Y), magenta (M) and cyan (C)), on one another in an appropriate combination on the same pixel. For example, for red (R), green (G), blue (B) and black (BK), basically as the combinations of the following expressions:Y+M→RY+C→GM+C→BY+M+C→Bk
Red (R) can be developed by appropriately mixing yellow (Y) and magenta (M), green (G) can be developed by appropriately mixing yellow (Y) and cyan (C), blue (B) can be developed by appropriately mixing magenta (M) and cyan (C), and black (Bk) can be developed by appropriately mixing yellow (Y), magenta (M) and cyan (C). However, when a black image is formed by overprinting inks of three primary colors (yellow, magenta and cyan), on one another, the volume of the ink droplets on a recording material becomes large. The line becomes thicker compared with portions where colors other than black are recorded resulting in an unnatural image, and ink absorption failure of the recording material occurs resulting in lowering of the drying property and the like.
In many cases, text and lines are formed in black ink, thus high optical density, sharpness and lightfastness are required for the black text and black lines. Often, therefore, a black ink is included for ink-jet recording in addition to color inks of yellow, magenta and cyan.
Since black ink has a high optical density per se, when a black image is formed only with a black ink, it is difficult to develop an image having gradations of medium lightness to high lightness, and in particular, it is difficult to reduce the graininess in a black image of high lightness.
Generally, “a black image of a medium lightness to a high lightness” means an image in dark gray to light gray such as an area with a lightness (L*) of about 30 or more and about 80 or less and with a chroma (C*) of about 50 or less.
In response to the problem of the image quality of such a black image of a medium lightness to a high lightness, a method using plural black inks having different densities as black inks has been proposed (U.S. Pat. No. 6,596,065). However, the use of plural black inks having different densities increases the number of inks constituting an ink set, resulting in increased cost and increased size of an ink-jet recording apparatus.